1. Field of the Invention
The present invention relates to a method of removing from a surface region of a silicon semiconductor substrate heavy metal impurities which are introduced into the substrate during wafer processing. More particularly, the present invention relates to a method of gettering heavy metal impurities, such as iron and chromium, which has been difficult to remove by a conventional intrinsic gettering method. As is known, if a CCD (charge coupled device) is used as an image sensor, heavy metal impurities introduced into the silicon substrate increase the level of an output signal of dark current, thus leading to a deterioration in sensor characteristics. The present invention is particularly effective when applied to gettering subsequent to wafer processing during CCD fabrication.
2. Description of the Related Art
In general, impurities such as a small amount of oxygen or carbon are introduced into silicon during the initial step of growing an ingot. It is, therefore, extremely difficult to avoid problems due to impurities, such as the contamination of wafers or the occurrence of crystal defects, in a device fabrication process using silicon wafers.
Gettering techniques are known for achieving good device characteristics by removing such impurities or incidental crystal defects from an element forming region. One gettering technique is called an IG (intrinsic gettering) technique in which impurity gettering is implemented by forming crystal defects in the interior of a silicon wafer and absorbing the impurities therein.
A typical example of the process of a known IG technique is explained below. In the process, prior to or in part during wafer processing, a silicon wafer is exposed to the following three steps of heat treatment in a nitrogen gas atmosphere.
Step (1) involves heat treatment of 30-60 minutes at 1100.degree. C. In Step (1), oxygen impurities are removed from a wafer surface by outward diffusion to form a defect-free zone on the surface. Step (2) involves heat treatment of several hours at 700.degree. C. In Step (2), nuclei of oxygen precipitation are produced in the interior of the wafer. In Step (3) involves heat treatment of 1 hour at 1000.degree. C. In Step (3), oxygen is attracted to the nuclei of oxygen precipitation and increase the number of associated small defects, thereby increasing the ability of gettering.
The high-density defect-layer region in the wafer in which oxygen is precipitated throughout the above-described steps, is called an IG layer and a surface defect-free zone is called a DZ (denuded zone).
Manufacturing processes for silicon semiconductor devices, particularly wafer processes, encounter the problem of contamination due to metallic impurities. The contamination due to metallic impurities may cause increases in leakage currents at pn junctions or an incidental deterioration in voltage-resistance characteristics, or otherwise the lifetime decrease of carriers. Impurities which have particularly serious influences are heavy metal impurities such as iron (Fe), chromium (Cr), copper (Cu) and nickel (Ni). In the case of an image sensor employing a semiconductor device such as a CCD, junction leakage currents due to impurities in a photodetector region increase dark current and decrease device performance. In DRAMs, increases in junction leakage currents will require a short refresh time and cause imperfect refreshing.
The above-described known IG technique which serves the function of removing such heavy metal impurities from an active element region, effectively works on Cu, Ni or the like, but is not sufficiently effective on Fe and Cr.
To improve the performance of semiconductor devices, it is desired to establish a reliable gettering technique for the above-mentioned heavy metal impurities.